The present disclosure relates generally to bearings and specifically to ball, roller and slider bearings, gears and seals, all having solid lubricants, occasionally in addition to a single initial charge only of lubricating grease, which can operate in adverse environments for long times without the necessity of repeated application of lubricating oil or grease. The present invention may be integrated with advanced gas-turbine and reciprocating internal-combustion engines, vehicular wheel bearings, railway tread-brake pivot bearings and high-speed motor technology, continuous fiber-reinforced composite or dry-filament wound flywheel ring and mounting-hub technology, and ultra-compact motor/generator and DSP-based electronic controller technology, for low-cost reliable gas-turbine and reciprocating engines, ultra-high-speed motors and long-life flywheel energy storage systems.
Ball, roller, sleeve and plain bearings are known and have been used in mechanical devices essentially since the beginning of the industrial revolution. While such bearings are serviceable in a wide variety of applications and environments, these conventional bearings have some disadvantages. One major disadvantage is the need for recirculating lubricating oil or periodic replenishment of grease to provide adequate lubrication between the moving surfaces, so that friction does not cause the bearings to overheat and self-destruct during operation. Another disadvantage in high-speed machines is the need to avoid excessive vibratory bearing loads as the rotor runs through critical speeds. An improved means to prevent water and abrasive contaminants from entering the bearings, such as for vehicular wheels, as well as a means for replenishing the solid-lubricant films on the contacting surfaces of gear teeth are desired.
In some applications such as high-speed flywheel energy storage systems, conventional bearings do not have sufficient life in the vacuum environment required to minimize windage losses and carbon-fiber composite flywheel rim overheating. To date such systems have relied on the use of expensive, energy-consuming, active magnetic bearings. Lubricating greases with the ultra-low volatility synthetic lubricating oils required in the high vacuum of such systems with lower cost ball bearings, do not have the additive response of synthetic hydrocarbons or conventional petroleum-based oils, so that they have limited boundary lubricating ability and bearing life.
Conventional petroleum-based oil and grease lubricants tend to dry out, oxidize and/or evaporate over time and generally must be replaced periodically for the bearing to function adequately over its design life. To perform such lubrication, workers require access to the bearings which means downtime for the associated machinery.
Adequately reliable solid lubricated, ultra low- or high-temperature bearing systems do not exist, especially in high-vacuum environments, even though such bearing systems would reduce maintenance and increase permissible operating temperature ranges and reliability of the associated machinery.
Use of solid lubricants and dry, wear-resistant materials in low and high temperature and high vacuum bearing applications has been attempted, using molybdenum disulfide, graphite and Teflon. The bearing industry markets several types of dry bearing materials based on such molybdenum disulfide, graphite, Teflon and other plastics to be used as solid lubricants. Woven glass fiber-reinforced Teflon bearings are fabricated by bonding a stiff metal backing to a thin composite layer of soft (but solid) lubricating Teflon, reinforced with a hard glass fabric so that a very thin film of Teflon lubricates the glass fibers with a minimum of deflection, plastic flow and wear. The current wisdom in the industry is that such solid-lubricant materials must be in continuous or nearly continuous contact with the surfaces to be lubricated like a lead-graphite pencil writing on paper, whereas this invention is counter to that intuitive concept, since so much less lubricating material is required to effectively lubricate the concentrated contacts in ball and roller bearings and gears, where we have measured successfully lubricating films at rolling contacts of only a few millionths of an inch thick by an X-ray method (see Sibley and Orcutt, “Elastohydrodynamic Lubrication of Rolling-Contact Surfaces,” ASLE Transactions, Vol. 4, 234-249, 1961), whereas the lubricating films in sliding contacts are a thousand times thicker at a few thousandths of an inch thick and thus require much more lubricating material to replenish.
In high-speed rotors operating above their critical speeds, the bearings are usually lubricated with circulating oil. In the case of high-speed ball or roller bearings, this lubricating oil is often circulated through an annular space in the housing that separates the non-rotating bearing ring from the main housing of the machine, so that the radial load on the bearing squeezes this oil film. The purpose of this squeeze film construction is to provide an elastic member in the form of the hydrodynamically-generated pressures in a thin oil film with viscous damping properties to separate the rotor from the machine supporting structure. Thus the vibrations of the rotor are dampened as it passes through its critical speeds either speeding up or slowing down from its normal super-critical operating speed.
Another common design for this purpose is to use elastomeric O-rings around the stationary bearing ring to separate the ring from the housing and thus provide the elastic compliance and damping required. However, the life of such O-rings under the high cyclic stresses in such applications is quite limited, and it is often not possible within the dimensional constraints available to obtain high enough damping to keep the vibratory bearing loads within acceptable limits.
For a self-contained lubrication system using the improved solid-lubrication configurations of this invention, or even a greased-for-life system which is now possible with these improved bearing designs at very high speeds, an alternative to the above-described oil squeeze film and O-ring dampers is needed to prevent excessive bearing vibratory loads at the critical speeds of machines with super-critical operating rotors without any damping losses. Important examples of such rotors are gas-turbine engines and the continuous-fiber-reinforced composite rings in flywheel energy storage systems (“FESS”).
In known solid-lubricated bearing applications, sporadic catastrophic bearing failures occur. Also, self-contained so-called greased-for-life bearings have limited high-speed capabilities and require frequent re-lubrication intervals. Reliable, self-contained, long-life solid-lubricated or grease-packed bearing designs are needed to reduce maintenance and to facilitate higher operating speeds and temperatures that can greatly improve the performance and energy efficiency of machines. In ultra-high-temperature applications, standard bearing steels do not retain sufficient hardness at temperature, so that all-ceramic bearings are required, and mounting devices are needed to support ceramic bearings on metal shafts and housings over wide temperature ranges without cracking such rings from their large differential thermal expansivity compared to metals.
When ceramic balls or rollers are used, it is often possible to eliminate the cage or retainer that separates these rolling elements, except for very high speeds or very large bearings, when the ball or roller inertial forces are high enough to cause damage at the ball/ball or roller/roller contacts, so that cages are then required. New designs of cages are needed to reduce as much as possible their interference with the motion of the balls or rollers and their high-shear damage to the lubricant films by skidding at the ball or roller-raceway contacts, and design guidelines are needed to compensate for roller skewing and skidding in solid-lubricated roller bearings. Also, cooling provisions are required to prevent excessive differential thermal expansions of the rings of solid-lubricated bearings.
The present disclosure contemplates a new and improved solid-lubricated bearing assembly that overcomes current limitations.